1. Field of the Invention
The present invention relates to an apparatus to control color registration and image density in a printer, and a method of calculating color registration error, and more particularly, to a color registration control apparatus to accurately detect color registration to provide a high quality picture, a method of calculating color registration error, and an apparatus to simultaneously detect both color registration and image density.
2. Description of the Related Art
Image forming apparatuses such as printers and copy machines form a latent electrostatic image by charging a photoconductive member on a transfer belt and performing selective exposure by scanning a laser beam, develop the latent electrostatic image using colored toners and a developer unit, and transfer the developed latent electrostatic image to a recording medium by pressing and heating, thereby forming an image.
Generally, the colors of toners used in a developer unit are cyan (C), magenta (M), yellow (Y), and black (K). The four color toners are transferred such that the four colors overlap to form a complete image. To deliver high quality images, unit images of individual colors should be accurately superimposed. This superimposition of colors is referred to as color registration.
Color registration errors can arise from complex causes such as mismatch of the individual color units of a developer unit, errors in processing an optical lens, and motion errors of a transfer belt. Particularly, color registration error becomes a problem in an image forming apparatus having a serial structure including a plurality of developer units.
Color registration errors have four types: X-offset, Y-offset, printing width error, and skew. X-offset arises in a scan direction in which a sensor scans. Y-offset arises in a cross-scan direction in which a transfer belt moves. Printing width errors arise from a difference in width of an image area. Skew arises from displacement of a development line. In order to obtain high quality images using color registration, a sensor to detect color registration errors and a method of accurately calculating the errors are required.
FIG. 1 is a diagram of a color registration sensor and a mark pattern disclosed in U.S. Pat. No. 5,287,162. Referring to FIG. 1, a color registration mark pattern 13 in a chevron shape is formed on a transfer belt (not shown). A split sensor 11 including two split cells 11a and 11b detects a beam reflected from the color registration mark pattern 13. Reference numeral 32 indicates a cross scan direction.
The split sensor 11 includes the two split cells 11a and 11b and is designed to compare the amount of light reflected from the color registration mark pattern 13 and detected by the cell 11a, with the amount of light reflected from the color registration mark pattern 13 and detected by the cell 11b, to produce an output when the quantities are the same.
The split sensor 11 must be aligned parallel to the color registration mark pattern 13, and formed in a chevron shape to detect different colored chevron marks. Accordingly, the split sensor 11 is expensive. In addition, when the split sensor 11 is not parallel to the color registration mark pattern 13, beams reflected from the color registration mark pattern 13 cannot be accurately detected.
FIG. 2 is a diagram of a color registration sensor and a mark pattern disclosed in U.S. Pat. No. 5,631,686. Referring to FIG. 2, a black swath 17 is laid on an intermediate transfer belt 19. Marks 25 corresponding to yellow, cyan, and magenta are formed on the black swath 17 in a chevron shape. A black mark 23 can be made by forming a void on the black swath 17. A bicell sensor 21 is provided above the intermediate transfer belt 19.
The bicell sensor 21 detects the marks corresponding to yellow, cyan, and magenta based on the difference between the low reflectivity of the black mark 23 and the high reflectivity of each of the marks of different colors, and detects the black mark 23 based on the difference between the low reflectivity of the black mark 23 and the high reflectivity of the intermediate transfer belt 19.
The bicell sensor 21 can easily detect the black mark 23 due to the great difference between the reflectivity of the black mark 23 and the reflectivity of the marks 25 corresponding to yellow, cyan, and magenta, but has difficulty detecting the marks 25 corresponding to yellow, cyan, and magenta since the difference in reflectivity between the marks 25 is small.
FIG. 3 is a diagram of a color registration sensor and a mark pattern disclosed in U.S. Pat. No. 5,909,235. Referring to FIG. 3, color registration mark sets 33 and 35 are provided at one side of a transfer belt 29. A wide area beam (WAB) sensor 31 to detect beams reflected from the color registration mark sets 33 and 35 is provided on the transfer belt 29. Here, reference numerals 72 and 34 denote image areas. Reference characters X and Y denote a scan direction and a cross-scan direction, respectively.
For each of the color registration mark sets 33 and 35, a plurality of black marks are formed first, and yellow, cyan and magenta marks are arranged in line with each of the black marks.
The WAB sensor 31 radiates beams at the color registration mark sets 33 and 35, measures the reflectivity of the color registration mark sets 33 and 35, and compares the areas of reflected beams with each other, thereby producing a detection signal. The WAB sensor 31 does not focus beams on the color registration mark sets 33 and 35, but diffusely radiates beams onto sets 33 and 35 to detect beams reflected therefrom.
Since the WAB sensor 31 diffusely radiates beams, all of the beams regularly and irregularly reflected from the color registration mark sets 33 and 35 are detected. Accordingly, detection errors having noise components may increase according to the surrounding conditions of the transfer belt 29. In addition, radiated beams have multiple wavelength bands, so the sensitivity of the WAB sensor 31 is not uniform for different wavelengths. As a result, the light receiving sensitivity of the WAB sensor 31 is not uniform, which decreases the accuracy of a detection signal.
In addition to color registration, i.e., arrangement of colors in juxtaposition, it is also necessary to appropriately adjust image density in order to obtain high quality images. Conventionally, a sensor such as a color registration sensor to detect color registration and a sensor such as a color toner density (CTD) sensor to detect image density are separate, so color registration error and image density error are separately detected and compensated for.
FIG. 4 is a diagram of a color registration sensor disclosed in U.S. Pat. No. 5,241,400. Referring to FIG. 4, charge coupled devices (CCDs) 40a and 40b are provided as color registration sensors. CCD drivers 48 and 49 are provided to drive the CCDs 40a and 40b. A registration adjuster 40 registers signals produced by the CCDs 40a and 40b. A system controller 41 receives a signal from the registration adjuster 40 and controls a system 45. A mode of a signal output from the system controller 41 is converted by a mode switching circuit 42, and is input into a driver 43. The driver 43 drives the system 45 according to the input signal.
The above conventional technique has drawbacks in that a pulse generator and a CCD driver are required to drive the CCDs in order to configure a system for detecting color registration and image density using the CCD, and the configuration of a signal detector is complicated and difficult since the levels of analog signals detected by the CCDs are different.
FIG. 5 is a diagram of an image density sensor disclosed in U.S. Pat. No. 6,115,512. Referring to FIG. 5, a white light source 58 to radiate a beam at marks (not shown) on a transfer belt 50 is provided at the center between light receiving devices 53 and 55. Filters 52 and 54 to selectively receive beams reflected from the marks on the transfer belt 50 according to color are provided on the front sides of the light receiving devices 55 and 53, respectively. The white light source 58 is a light emitting diode (LED). A beam radiated from the white light source 58 should have a wide area in order to sufficiently illuminate the marks. Accordingly, the detection area of each of the light receiving devices 53 and 55 should be wide. Since the spot size of the beam radiated from the LED white light source 58 used in the above conventional technique is large, detection errors in color registration are large, making it difficult to detect color registration.
FIG. 6 is a diagram of an image density sensor disclosed in U.S. Pat. No. 5,303,037. Referring to FIG. 6, a red light source 65 is provided at the center between a blue light source 63 and a green light source 67. An acrylic prism 69 directs the beams output by the blue, red and green light sources 63, 65 and 67 into a focusing lens 64 which focuses the beams onto a mark (not shown) on a transfer belt 62. Since the above-described conventional technique uses the three color light sources 63, 65, and 67 for an image density sensor, the configuration of the sensor is complicated by the need to maintain a constant light output from the light sources 63, 65, and 67, making the sensor expensive.
As described above, conventional image forming apparatuses are provided with separate sensors for color registration and image density. They do not use a sensor performing both functions together. In addition, conventional color registration sensors have complicated configurations and poor performance, so it is difficult to accurately detect color registration error.